Field of the Invention
This invention relates to a ventilation apparatus for artificial respiration which is adapted to supply respiratory gas to a patient suffering from respiratory embarrassment to effect artificial respiration.
As is well known, respiration or breathing is gas exchange by which oxygen is supplied to a living body (patient) while carbon dioxide is expelled from the living body. The lungs which function as organs of respiration consist essentially of an inverted tree of intricately branched bronchioles that communicate with thin-walled terminal alveoli swathed in a network of delicate capillaries. The gas exchange of respiration takes place between the capillaries and the alveoli into which the air is inspired. The gases subjected to the gas exchange are expelled to the atmosphere by the diffusion of the gases in the fine brochioles into which the trachea is ramified more than eighteen times, and the gas expelling is also effected through the ventilation achieved by the alternate deflation and inflation of the lungs in the relatively large bronchioles into which the trachea is ramified not more than eighteen times.
Patients suffering from respiratory embarrassment need to be subjected to artificial respiration. Various artificial respiration methods have been known in the art. These respiration methods may be broadly divided into two classes. A respiration method of one class is to assist in the above ventilation effected by the alternate deflation and inflation of the lungs. This method has long been used and is to intermittently maintain all the bronchioles and bronchi at a positive pressure so as to assist in the deflation and inflation of the lungs, thereby causing a patient to inspire and expire a respiratory gas. A typical example is an intermittent positive pressure ventilation (IPPV) method. Intermittent positive pressure ventilators for performing this method have been proposed and used extensively.
A respiration method of the other class is to apply oscillations at a frequency of 200 to 1500 cycles per minute to the lungs so that the partial pressures of O.sub.2 and CO.sub.2 in the blood are kept to a proper level even during apnea. This method has been proposed recently and is referred to as a high frequency oscillation (HFO) ventilation method in the trade. High frequency oscillation ventilators for performing this method have already be proposed and used.
As is well known, natural respiration or breathing is effected by inflating the lungs to maintain the bronchioles and bronchi at a negative pressure to inspire air and deflating the lungs to maintain the bronchioles and bronchi at a positive pressure to expire air. A patient suffering from respiratory embarrassment can not totally or adequately inflate and deflate the lungs for respiration. The respiratory embarrassment may be divided to various stages and broadly to two stages, i.e., (a) the stage in which the patient is completely under apnea and (b) the stage in which the patient can breathe by himself though inadequately.
The intermittent positive pressure ventilator has been quite frequently used to save patients at the above stage (a) of respiratory embarrassment. However, with the intermittent positive pressure ventilator, the lungs are intermittently maintained at a positive pressure to inspire a respiratory gas into the bronchioles, bronchi and alveoli in contrast with the natural respiration. As a result, the intermittent positive pressure ventilator has suffered from the following disadvantages:
(i) The intermittent positive pressure ventilator can only be used effectively for a patient at the above stage (a) of respiratory embarrassment and is very difficult to be used for a patient at the above stage (b) of respiratory embarrassment particularly where the patient is a baby or a child. The reason for this is that it is difficult to supply the respiratory gas through the ventilator to the respiratory tract of the patient accurately in synchronism with the inspiration of the patient. As a result, there are often occasions when the respiratory gas is supplied to the lungs of the patient during the expiration of the patient. This makes it more difficult for the patient to respire. In addition, this offers resistance to the respiratory action of the patient to cause pain to the patient.
(ii) Another disadvantage of the intermittent positive pressure ventilator is that it tends to compress the blood vessels in the thorax of the patient and disturb the blood flow so that the patient is subjected to pressure damage such as pneumothorax and suffers from the decreased heart rate. Thus, an undue load is applied to the lungs and heart of the patient. This is quite dangerous particulary when the intermittent positive pressure ventilator is applied to the patient under a serious condition who is suffering from respiratory embarrassment. The reason for the above undesirable phenomena is as follows: The ventilator of this type is designed to forcibly inspire respiratory gas intermittently into the respiratory tract of the patient so that the respiratory tract is maintained intermittently at a positive pressure. So long as the thorax of the patient is not deflated and inflated by his own respiratory action, the thorax always tends to contract. Therefore, while the respiratory tract (particulary the alveoli) is maintained at a positive pressure, the blood vessels adjacent to the alveoli are sandwiched between the alveoli and the outer portion of the thorax and are subjected to undue pressure.
In order to overcome the above disadvantage (i), there has been proposed an intermittent positive pressure ventilator of the jet type (known as a jet ventilator) in which a jet of respiratory gas is sent to the respiratory tract of the patient through a fine tube having a diameter of about 1 mm so that the supply of respiratory gas may be effected in synchronism with the natural respiration of the patient. Actually, however, this synchronism could not satisfactorily be achieved.
The high frequency oscillation ventilator mentioned above is designed to apply oscillations at a frequency of 100 to 5000 cycles per minute to the lungs. This ventilator causes the patient to respire without the inflation and deflation of his thorax. Therefore, in contrast with the intermittent positive pressure ventilator, the high frequency oscillation ventilator can supply respiratory gas to the patient without unduly maintaining the lungs at a positive pressure. Therefore, the high frequency oscillation ventilator can be used for patients both at the above stages (a) and (b) of respiratory embarrassment, and does not suffer from the above disadvantages of the intermittent positive pressure ventilator. Thus, the high frequency oscillation ventilator has such excellent advantages, but can not always be applied effectively in all situations. For example, when the high frequency oscillation ventilator is used for a patient at the above stage (a) of respiratory embarrassment, this ventilator can not achieve satisfactory results. In order to overcome this difficulty, it is necessary to apply respiratory gas to the respiratory tract to a certain degree. Therefore, the respiratory tract need to be subjected to a gas pressure (positive pressure) either continuously or intermittently to such a degree that this gas pressure is not a burden to the patient.
There have been proposed various types of high frequency oscillation ventilators and intermittent positive pressure ventilators which include those using a piston and those using a pneumatic circuit. However, such conventional ventilators are so constructed that the ventilation rate is not variable. As a result, such conventional ventilators have failed to provide a proper respiratory treatment depending upon the condition of respiratory embarrassment. Further, the conventional high frequency oscillation ventilators use a flow control valve (throttle valve) or the like for controlling the flow rate of a signal gas so as to control the supply of respiratory gas to the patient at a predetermined interval. However, the leading and trailing edges of the waveform of the signal become non-linear so that the control of the pulse width is not carried out properly. As a result, the supply and interruption of the respiratory gas to the patient can not be achieved at a regular interval.